Calculation on cylinder pressure fluctuation by using the wave equation in KIVA program

Cylinder pressure fluctuation during combustion process of internal combustion engine is closely related to combustion noise and knock.The current studies are based on cylinder pressure test to obtain ...     

Numerical investigation on effects of rivulet and cable oscillation of a stayed cable in rain-wind-induced vibration

Rain-wind-induced vibration (RWIV) appeared on cable stayed bridges involves complicated fluid and structure interactions and its mechanism is not fully understood. It is believed that the upper-rivulet which is often seen when the RWIV occurs plays an important role. In this paper, a numerical investigation on the effects of the upper rivulet on the aerodynamic forces of the cable and the interaction between the fluid flow and the cable oscillation is carried out where the cable with the upper-rivulet is modeled by a circular cylinder with an arch attachment. The Reynolds number of 6.8×10⁴ is selected. The large-eddy simulation (LES) method with Smagorinsky-Lilly modeling is employed to simulate the 3-D turbulent flow field, and a moving mesh method is introduced to deal with the oscillation of the cylinder. The aerodynamic forces on the cylinder and the flow patterns around the cylinder are analyzed for both steady and oscillation status of the cylinder with the rivulet attachment in different position angle ranged from 0° to 60°. The results show that the aerodynamic forces on the cylinder change largely with the position angle of the rivulet attachment. In the steady cases, a uniform recirculation flow along the axial direction of the cylinder forms behind the cylinder; whereas in the oscillation cases, a 3-D periodical recirculation flow appears along the axial direction. The scales of these recirculation zones are influenced by both the position angle of the rivulet and the dynamic status of the cylinder. The results also show that there exists a rivulet position angle where the aerodynamic force on the cable and flow pattern around the cable both change dramatically. This critical position angle is found to be 45° for the cases studied.     

Numerical investigation of cross-flow around a circular cylinder at a low-reynolds number flow under an electromagnetic force

The effect of the electromagnetic force (or Lorentz force) on the flow behavior around a circular cylinder is investigated by computation. Two-dimensional unsteady flow computation forR _e =10² is carried out using a numerical method of finite difference approximation in a curvilinear body-fitted coordinate system by solving the momentum equations including the Lorentz force as a body force. The effect of spatial variations of the Lorentz forcing region and forcing direction along the cylinder circumference is investigated. The numerical results show that the Lorentz force can effectively suppress the flow separation and oscillation of the lift force of circular cylinder cross-flow, leading to reduction of drag.     

串列三圆柱绕流的时均压力分布与气动力

运用刚性模型测压风洞试验方法对单圆柱、不同间距串列双圆柱和串列三圆柱绕流的时均压力分布与气动力进行了研究。首先,进行单圆柱模型和不同间距串列双圆柱模型的绕流试验,试验的雷诺数为3.4×104;其次,通过与单圆柱进行对比,讨论了气动干扰对串列三圆柱时均压力分布与时均阻力的影响规律;最后,通过与串列双圆柱进行对比,讨论了圆柱的数量对干扰规律的影响。试验结果发现,串列三圆柱的绕流存在两个完全不同的流态,其切换的临界间距(L/D)cr在3.5～4.0之间,两个流态下的时均压力分布与时均阻力存在明显的差异。本研究可对实际工程中串列圆柱结构的风荷载取值提供参考。     

Fundamental studies on free stream acceleration effect on drag force in bluff bodies

This paper describes fundamental studies on free stream acceleration effect on drag force in bluff bodies. The flow with gradual velocity increase assumed an accelerated flow. The wind tunnel tests were conducted in order to investigate the difference of aerodynamic characteristics between non-accelerated flow and accelerated flow. The experimental models were a circular cylinder and a square cylinder. In an accelerated flow, the condition of free stream was an acceleration of about 3.6m/s². Experimental Reynolds number varied between form 4.0×10⁴ to 1.64×10⁵. The pressure distributions and the aerodynamic force were measured in both case of nonaccelerated flow and accelerated flow. In case of a circular cylinder, the drag of accelerated flow is lower than that of non-accelerated flow in the low Reynolds number regime. Then, it becomes higher than that of non-accelerated flow in the high Reynolds number regime. On the other hand, in case of a square cylinder, the drag of accelerated flow is higher than that of non-accelerated flow in the whole Reynolds number regime. If a separation point can be movable such as the circular cylinder, the additional momentum due to flow acceleration leads to delay separation and to decrease the drag in the low Reynolds number regime. If a separation point is nearly fixed such as a square cylinder, the additional momentum due to acceleration always affects to increase the drag than that of non-accelerated flow. Based on this research, it is expected that the roof shape of high speed train where the separation point is movable has the advantages to reduce the effect of accelerated flow such as strong crosswind/gust.     

Aeroacoustic characteristics and noise reduction of a centrifugal fan for a vacuum cleaner

The aeroacoustic characteristics of a centrifugal fan for a vacuum cleaner and its noise reduction method are studied in this paper. The major noise source of a vacuum cleaner is the centrifugal fan. The impeller of the fan rotates at over 3000 rpm, and generates very high-level noise. It was revealed that the dominant noise source is the aerodynamic interaction between the rotating impeller and stationary diffuser. The directivity of acoustic pressure showed that most of the noise propagates backward direction of the fan-motor assembly. In order to reduce the high tonal sound generated from the aerodynamic interaction, unevenly pitched impeller and diffuser, and tapered impeller designs were proposed and experiments were performed. Uneven pitch design of the impeller changes the sound quality while the overall sound power level (SPL) and the performance remains similar. The effect of the tapered design of impeller was evaluated. The trailing edge of the tapered fan is inclined. This reduces the flow interaction between the rotating impeller and the stationary diffuser because of some phase shifts. The static efficiency of the new impeller design is slightly lower than the previous design. However, the overall SPL is reduced by about 4 dB(A). The SPL of the fundamental blade passing frequency (BPF) is reduced by about 6 dB(A) and the 2^nd BPF is reduced about 20 dB(A). The vacuum cleaner with the tapered impeller design produces lower noise level than the previous one, and the strong tonal sound was dramatically reduced.     

The direct and inverse problems of an air-saturated porous cylinder submitted to acoustic radiation

Gas-saturated porous skeleton materials such as geomaterials, polymeric and metallic foams, or biomaterials are fundamental in a diverse range of applications, from structural materials to energy technologies. Most polymeric foams are used for noise control applications and knowledge of the manner in which the energy of sound waves is dissipated with respect to the intrinsic acoustic properties is important for the design of sound packages. Foams are often employed in the audible, low frequency range where modeling and measurement techniques for the recovery of physical parameters responsible for energy loss are still few. Accurate acoustic methods of characterization of porous media are based on the measurement of the transmitted and/or reflected acoustic waves by platelike specimens at ultrasonic frequencies. In this study we develop an acoustic method for the recovery of the material parameters of a rigid-frame, air-saturated polymeric foam cylinder. A dispersion relation for sound wave propagation in the porous medium is derived from the propagation equations and a model solution is sought based on plane-wave decomposition using orthogonal cylindrical functions. The explicit analytical solution equation of the scattered field shows that it is also dependent on the intrinsic acoustic parameters of the porous cylinder, namely, porosity, tortuosity, and flow resistivity (permeability). The inverse problem of the recovery of the flow resistivity and porosity is solved by seeking the minima of the objective functions consisting of the sum of squared residuals of the differences between the experimental and theoretical scattered field data.     

Effect of rounding on flow-induced forces on a square cylinder

A numerical study has been carried out to elucidate the effects of rounding the sharp edges on flow-induced forces on a square cylinder immersed in a laminar cross flow. Rounding reduces both the upstream stagnation pressure and the downstream base pressure. Consequently, competition between these two pressure reductions yields the minimum drag on the cylinder when its edges are partially rounded. It was also found that the leading-edge rounding is mainly responsible for the topological change thus the drag reduction, while the trailing- edge rounding alone just enhances lift fluctuation. However, trailing-edge rounding plays a role of stabilizing the flow when all of the four edges are rounded.

     

Pressure control of hydraulic servo system using proportional control valve

The purpose of this study is to develope a control scheme for the hydraulic servo system which can rapidly control the pressure in a hydraulic cylinder with very short stroke. Compared with the negligible stroke of the cylinder in the system, the flow gain of the proportional pressure control valve constituting the hydraulic servo system is relatively large and the time delay on the response of the valve is quite long. Therefore, the pressure control system, in this study tends to get unstable during operations. Considering the above mentioned characteristics of the system, a two-degree-of-freedom control scheme, composed of the I-PDD²… feedback compensator and the feedforward controller, is proposed. The reference model scheme is used in deciding the parameters of the controllers. The validity of the proposed control scheme is confirmed through the experiments.     

Two-Dimensional Piston Ring Lubrication—Part II: Elastic Ring Consideration

A two-dimensional analysis for piston ring lubrication is presented by considering elastic deflection, EHL and cavitation effects. A numerical procedure is developed for solving hydrodynamic pressure and oil film, thickness shape due to ring deflection and elastic deformation. An elliptic cylinder liner and elastic ring are considered to investigate the circumferential flow effect which has been ignored in previous studies. Results for a typical automotive engine demonstrates that the elastic deflection and deformation of the piston ring have a tendency to reduce the gap caused by the noncircular cylinder. Under the high combustion chamber pressure, the minimum film thickness tends to have a uniform value in the circumferential direction. In turn, this provides a level of control of circumferential flow when the combustion chamber pressure is low. However, the elastic deflection of the piston ring always creates a potential for large blow-by around the piston ring gap area when the cylinder is non-circular. The two-dimensional analysis presented also reveals a reduction of piston ring oil film thickness due to the circumferential flow compared to one-dimensional analysis.     

Analysis of vortical flow field related to aero-acoustic sound in an air conditioning system by wall pressure measurement and CFD (1st report,resonance in a duct)

This paper elucidates three-dimensional flow phenomena that induce aero-acoustic noise at certain frequencies. Three-dimensional separated and vortical flow fields in an air conditioning system are investigated by Experimental fluid dynamics (EFD) analysis using an unsteady wall pressure measurement system with 30 high-respond pressure transducers and by Computational fluid dynamics (CFD) analysis using a Reynolds-averaged Navier-Stokes (RANS) simulation with a k-ε turbulence model. The EFD investigation revealed that the regions with high wall pressure fluctuation are located near the scroll tongue and on the bottom of the diffuser casing wall. A wall pressure fluctuation of 0.056 Blade passing frequency (BPF) was observed at sensor A near the scroll tongue, and a wall pressure fluctuation of 0.173 BPF was observed on part of the evaporator casing wall. The CFD investigation revealed that the reverse flow is generated by interaction between the scroll tongue and multi-blade fan. The reverse flow develops strongly over a wide range in the multi-blade fan near the scroll tongue. Furthermore, there is also a longitudinally separated vortex near the bottom of the diffuser casing. The reverse flow and longitudinally separated vortex interact with the wall casing. For this reason, the scroll tongue and the bottom of the diffuser casing had high pressure fluctuations in the EFD investigation. By considering aero-acoustic factors, an aerodynamic sound of 0.056 BPF can be explained by Helmholtz resonance phenomenon, which is generated by high pressure fluctuation due to the reverse flow near the scroll tongue. An aerodynamic sound of 0.173 BPF can be explained by the standing wave phenomenon, which is generated by high pressure fluctuation due to the longitudinally separated vortex on the bottom of the diffuser casing wall.

     

Drag reduction for flow past a square cylinder through corner chamfering

This study investigates the unsteady incompressible flow around a square cylinder with different chamfer ratios (CRs) using a commercial finite volume code, ANSYS Fluent. CR ranges from 0.0 (sharp square cylinder) to 0.5 (diamond cylinder) with variable increments. Detailed analysis of flow characteristics is conducted at Reynolds number (Re) = 2100. Additionally, simulation is extended to cover Re, i.e., Re = 100, 500, and 10000. The simulation results show that cylinder with CR = 0.1 outperforms all other cases by enabling a drag reduction of about 60 % at Re = 10⁴. Drag has an inverse relationship with the wake closure length. Time-averaged coefficient of pressure, streamlines, and vorticity contours are also discussed to better understand near-wake features and the physics of drag reduction.

     

旋转锯片空气动力学噪声特性计算与分析

为分析旋转刀具在气流作用下的声学特性,基于多场耦合计算方程建立了流场-结构-声场联合仿真模型,通过弹性体与流场的动力学计算得到了结构强迫振动响应,利用边界元法将振动响应信息作为声学边界,获取了锯片的声场辐射信息,噪声预估值与实测结果吻合良好。研究了锯片旋转过程中周围流场媒介的层流、湍流特性对锯片振动的影响规律;揭示了流-固耦合作用对声场时域、频域影响作用机制;确定了锯片空载噪声辐射的方向特性;分析了锯片尺寸结构对噪声的影响规律,为低噪声锯片的设计提供了参考依据。     

Flow structure in the downstream of square and circular cylinders

In this experimental work, a technique of digital particle image velocimetry (DPIV) is employed to characterize instantaneous vorticity and time-averaged velocity, vorticity, root mean square (rms) velocities, Reynolds stress correlations and phase-averaged contours in the downstream of circular, sharp-edged square and 45^∘ orientated square cylinders in a uniform flow. Strouhal numbers for 550≤Re≤3400 are calculated from wake flow patterns. Shear layers surrounding the recirculation bubble region behind the cylinder are discussed in terms of flow physics and vortex formation lengths of large-scale Kármán vortices. Enhancement levels of Reynolds stress correlations associated with cross-stream velocity are clarified. Finally, flow structures depending on the cylinder geometry and Reynolds number are interpreted with quantitative representations.     

Effect of suction nozzle modification on the performance and aero-acoustic noise of a vacuum cleaner

The suction nozzle of a vacuum cleaner was modified to enhance the power performance and to reduce the airflow-induced acoustic noise. The suction power efficiencies of the vacuum cleaner were measured for various nozzles: (1) original nozzle, (2) original nozzle with modified trench height, (3) original nozzle with modified connecting chamber, and (4) a combination of (2) and (3). In addition, the suction pressure and sound pressure level around the suction nozzle were measured to validate the reduction of acoustic noise. The power efficiency and mean suction pressure increased when the trench height of the suction nozzle was increased. This was attributed to the suppression of the flow separation in the suction channel. Modification of the connecting chamber in the original nozzle, which had an abrupt contraction from a rectangular chamber into a circular pipe, into a smooth converging contraction substantially improved the suction flow into the connecting pipe. When both modifications were applied simultaneously, the resulting suction nozzle was more effective from the viewpoints of aerodynamic power increase and sound pressure level reduction.     

Pressure field estimation from ultrasound Doppler velocity profiler for vortex-shedding flows

A novel algorithm of pressure field estimation based on ultrasound velocity profiler (UVP) is developed. The method consists of UVP measurement of velocity distribution in fluid flows and numerical analysis of the measured data using fluid dynamics equations. We introduce equation of continuity, incompressible Navier-Stokes equation and proper orthogonal decomposition (POD) into the basic algorithm, so that pressure field of space-time two-dimensional unsteady fluid flow is fully reconstructed. Since UVP is based on ultrasound Doppler principle, the local instantaneous pressure distribution is obtained non-intrusively. The performance of an algorithm is evaluated for vortex shedding flow behind a circular cylinder at Re = 1000. Considering the specification of UVP, the optimal method of experimental data conversion to pressure information is proposed. We have found that the one-dimensional velocity measurement by UVP upon Taylor's frozen hypothesis is suitable for evaluation of pressure field in wake of the cylinder. The present algorithm is also demonstrated for opaque fluid flows by considering vortex flow in milk.     

Stability of confined thin-walled steel cylinders under external pressure

The present paper investigates the structural stability of thin-walled steel cylinders surrounded by an elastic medium, subjected to uniform external pressure. A two-dimensional model is developed, assuming no variation of load and deformation along the cylinder axis. The cylinder and the surrounding medium are simulated with nonlinear finite elements that account for both geometric and material nonlinearities. Cylinders of elastic material within a rigid boundary are considered first, and the numerical results are compared successfully with available closed-form analytical predictions. Subsequently, the external pressure response of confined thin-walled steel cylinders is examined, in terms of the initial out-of-roundness of the cylinder, the initial gap between the cylinder and the medium, and the stiffness of the surrounding medium. Numerical results are presented in the form of pressure-deformation equilibrium paths, and show a rapid drop of pressure after reaching the maximum pressure level, as well as a significant imperfection sensitivity. A plastic-hinge mechanism is developed that results in a closed-form expression and illustrates the post-buckling response of the cylinder in an approximate manner. The distributions of plastic deformation, as well as the variation of cylinder-medium contact pressure around the cylinder cross-section are also depicted and discussed. Furthermore, the effects of uniform vertical preloading on the maximum pressure sustained by the cylinder are examined. Finally, the numerical results show good comparison with a simplified closed-form expression, proposed elsewhere, which could be used for design purposes.     

Numerical study of fluid force reduction on a circular cylinder using tripping rods

A numerical investigation on the effects of small tripping rods on the fluid force reduction on a big structure has been carried out using finite volume method where a configuration of a circular cylinder with two small tripping rods symmetrically placed very near to its front surface is studied. The diameter ratio of the rods and the cylinder is set at 0.08, 0.10 and 0.12, and the gap between the rods and the cylinder is fixed at 0.08 of the cylinder diameter. The angular position of the rods varies from 20° to 60°. The effects of the tripping rods on force reduction, vortex shedding frequency and flow separation have been examined for various arrangements of the rods with Reynolds number focused on 200 for laminar flow and 5.5×10⁴ for a turbulent flow. The results reveal that there exits an optimum position where the time averaged force coefficients acting on the cylinder all reach their minimum values and at the same time Strouhal number meets its maximum. At the optimum position the drag coefficient is reduced by 18% for Re=200 and 59% for Re=5.5×10⁴. Further investigation with tripping rods placed near the separation points is also carried out for Re=5.5×10⁴ and a considerable drag reduction is found.     

Drag force control of flow over wavy cylinders at low reynolds number

Three-dimensional numerical simulations on the laminar flow around a circular cylinder with different diameter along the spanwise leading to a type of sinusoidal waviness, named wavy cylinder are performed at low Reynolds number. A series of wavy cylinders with different combinations of spanwise wavelength and wave amplitude are conducted at a Reynolds number of 100. The optimal range of wavelength and the effect of wave amplitude are obtained. The results show that the 3-D free shear layers from the cylinder are more difficult to roll up to vortex and hence the wake formation lengths of some typical wavy cylinders are larger than that of the circular cylinder and in some cases the free shear layers even do not roll up into vortex behind the cylinder. The mean drag coefficients of the typical wavy cylinders are less than that of a corresponding circular cylinder with the same mean diameter; also the fluctuating lift coefficients are reduced. The reduction of mean drag coefficient and fluctuating lift coefficient of wavy cylinder increases with the value of wavy amplitude. Furthermore, a typical wavy cylinder model at Re=150 is also simulated and found that the control of flow induced vibration by modifing the spanwise wavelength of cylinder has a relationship with the variation of Reynolds number.     

A numerical study on mixed convection in a lid-driven cavity with a circular cylinder

A two-dimensional numerical simulation is carried out in this study to investigate mixed convection in a lid-driven cavity with an isothermal circular cylinder. The simulation is conducted at three Reynolds numbers of Re = 100, 500, and 1000 under a fixed Grashof number of Gr = 10⁵. The top wall of the cavity moves to the right at a constant velocity and is kept at a low temperature of T _c , whereas the stationary bottom wall is kept at a constant high temperature of T _h . The immersed-boundary method, which is based on the finite volume method, is adopted for the boundary of the circular cylinder that is present in the square cavity. The present study aims to investigate the effects of circular cylinder on fluid flow and heat transfer in a cavity at different locations. The fluid flow and heat transfer characteristics in the cavity strongly depend on the position of the circular cylinder as well as on the relative magnitude of the forced convection and the natural convection caused by the movement in the top wall of the cavity and the heating at the hot bottom wall, respectively.